In his character novel Master of Mankind, Games Workshop pretty much gave us the closest confirmation of his original appearance as we're going to get from them, considering this description is told during his penultimate "Big Damn Hero" moment, where it's implied he's letting his mask slip

And in a sunless realm, the sun rose at last.

The light of dawn was palpable on Ra’s armour as well as his skin. It was a pressure, a presence with searing physicality. The enemy hordes felt it as acid on their skin. The creatures – daemons no matter what secular truths held strong – lost what little order they had ever possessed.

The Anathema! Ra heard their frantic agony as a sick scraping on the edges of his mind. The Anathema comes! The sun rises!

His features were those of one born in the wild lands of Ancient Eurasia. His skin was a Terran blend of bronze and burnt umber, His eyes darker still, His hair darkest of all. The long black fall of His hair was held by a simple circlet crown of metal leaves, binding the mane back from His face so He could fight. More practical than regal.

I wonder if his true form, while he was alive, was like Mr. House's true form from Fallout New Vegas.